Private premise-based telephone systems, such as those installed at correctional facilities or other controlled-environment facilities, have created a need to monitor various events occurring on the telephone lines of the system. Telephone systems at correctional facilities or other controlled-environment facilities may comprise a microprocessor-based call processing system having operational software that is capable of allowing control over telephones connected to the system. For example, the system may be programmed to prevent inmates or residents from contacting unauthorized parties or using the telephone system for fraudulent purposes. An authorization mechanism may be utilized to prevent residents from dialing unauthorized numbers directly.
Additionally, a call processing system my prevent a resident from initiating a three-way call, taking part in a conference call, or the like. However, a particular problem that is encountered in these systems is the placement of a three-way call, or the like, by a party that is authorized to be called by the resident. Once the resident is connected to an authorized number, the resident may be connected to a third party at an unauthorized number via the three-way call feature by a party at the authorized number. Care may be taken to insure that a resident does not call an unauthorized party. However, once a call is connected through the Public Switched Telephone Network (PSTN) it becomes very difficult to control the actions of the called party. Therefore, to preserve this screening activity, it is desirable to insure that the called party is in fact the person to whom the call is terminating. Therefore, it is desirable to have control of the call with respect to all the parties who are on the phone call. In short, it is desirable to prevent addition of an unknown third party to a resident call in order to preserve the integrity of the initial call screening
A three-way call may be initiated when the originally called party (e.g. an authorized party outside the private telephone system) depresses the hook switch on the telephone, generating a hook flash signal. This signals the telephone central office to put the resident on hold and provide a dial tone to the originally called party. On receipt of the dial tone, the originally-called party dials the number of an unauthorized third party, and when the connection is completed, the resident and the unauthorized third party can communicate through the connection established outside the private system.
Three-way call monitoring systems which have been developed to prevent unauthorized calls according to the foregoing scenario rely on the detection of telephone signals. They typically monitor the local telephone connection for the hook flash “click” signal or associated central office signals that fall in a frequency band outside the range of frequencies produced by the human voice. These systems typically monitor signals on the local telephone line through a frequency filter designed to pass audio signals in this frequency band. A three-way call attempt may be indicated whenever signals in the frequency band have energies above a selected threshold. Some systems compare the signals with a hook flash reference signal utilizing sampling techniques implemented with a digital signal processor (DSP).
Even in a more or less conventional telephone environment these systems may not be very accurate for a number of reasons. The underlying assumptions about the frequency profile of three-way call events, i.e. the hook flash and signals generated by activating central office switches, are often wrong. For example, the hook flash signals are often modified by transmission through switches and along loaded lines, and even if assumptions about the frequency characteristics of the initial signal are accurate, these characteristics may be substantially distorted by the time the “hook flash” signal reaches a call processing system implementing three-way call detection, or the like.
Other systems and methods for detecting undesired call activity are disclosed in Salibrici, U.S. Pat. No. 5,768,355 and above-incorporated commonly owned, co-pending U.S. patent application Ser. No. 10/252,956, filed Sep. 20, 2002 and entitled THREE-WAY TELEPHONE CALL PREVENTION SYSTEM AND METHOD. Salibrici teaches using digital signal processing to identify a third-party connection. Salibrici operates by establishing a baseline ambient, or background, noise level, and detecting when the signal noise level drops below the ambient noise level. When the current signal noise level drops below the ambient noise level, the system assumes that a three-way conference call has been attempted by the called party. U.S. patent application Ser. No. 10/252,956 discloses an exemplary technique for detecting three-way calls, which in general includes detecting a call signal level, determining if the call signal level is below a predetermined silence level threshold, and measuring a duration the call signal level remains below the predetermined silence level threshold.
Internet protocol (“IP”) is a routing protocol designed to route traffic within a network or between networks. VoIP is a method for providing voice capabilities over an IP network, such as the Internet or an intranet. In such networks data packets are sent to and from communication sites to facilitate communication. In communication systems utilizing a VoIP protocol, the packets are commonly referred to as datagrams. In typical VoIP networks, each communication site sends datagrams to other communication sites with which they are in communication. There are different approaches to sending datagrams. Control signals per ITU recommendation H.323, and audio-based media streams using Real-Time Transport Protocol (RTP) per Internet RFC 1889, may be applied. Alternatively, control signals could be applied using other protocols such as Session Initiation Protocol (SIP) per Internet RFC 2543.